I'm getting System.Exception with HResult=0x800704C9 when there is a pause between connecting and starting SSL handshake (so the remote server closes the connection). await socket.UpgradeToSslAsync(SocketProtectionLevel.Tls12, socket.Information.RemoteHostName); Windows.Networking.Sockets.SocketError.GetStatus(e.HResult) gives Unknown for 0x800704C9 code. Error Lookup tool, however, recognizes this code and indicates that 0x04C9/0x800704C9 codes correspond to "The remote computer refused [...] read more